Conventionally, in order to join an electroconductive ceramics with an electroconductive ceramics or a metal, a method by a direct resistance heating, a method by a high frequency inductive heating, or a combination of these methods etc. have been proposed. According to the aforementioned method by a direct resistance heating, when an body to be jointed which is composed of two materials to be jointed parts at either side of a butted portion is arranged in contact with an electrode and supplied an electric current, a Joule heat is generated at the butted portion thereby to partially heat the butted portion. Therefore, a joining agent at the butted portion is melted, whereby the to be jointed parts are jointed together. According to the method by a high frequency inductive heating, an induction coil is provided around the butted portion of the body to be jointed, and the butted portion is partially heated by Joule heat resulting from the induction heating, so that the joining agent of the butted portion is melted. Meanwhile, when the method by a direct resistance heating is used in combination with the method by a high frequency inductive heating, a ceramics of a higher resistance, namely, having a lower electrical conductivity is heated beforehand by the high frequency inductive heating to lower the resistance value (that is, to increase the conductivity). Thereafter, the direct resistance heating is carried out to flow a large amount of current to quickly heat the butted plane.
FIG. 11(A) shows the structure when the conventional method by a direct resistance heating is performed. In the structure of FIG. 11(A), conductive cylindrical ceramics 1a', 1b' to be jointed are brought to butt against each other via a joining agent 3', thereby constituting a butted portion. Ring-shaped electrodes 2a', 2b' are provided butting against the corresponding ceramics 1a', 1b' in a manner as indicated in the drawing. Thermal insulators 4a', 4b' are placed at the end parts of the ceramics 1a', 1b', respectively. A voltage is impressed between the electrodes 2a' and 2b' while the ceramics 1a', 1b' are pressured by a pressuring device (not shown) via the thermal insulators 4a', 4b'. As a result, Joule heat is generated at the ceramics 1a', 1b' because of the flow of a current in a vertical direction to the butted plane of the ceramics 1a', 1b'. Since the butted plane is partially heated by this Joule heat, the joining agent 3' is melted, thereby joining the ceramics 1a', 1b' together. In such a case as above where the materials to be jointed are made of the same material in the same shape and jointed with use of electrodes of a small heat capacity, since the resistance values of both materials are equal to each other, the quantities of heat generated at materials to be jointed between the electrodes 2a', 2b' become equal to each other. Accordingly, the two materials to be jointed in the vicinity of the butted portion are heated generally uniformly as is indicated by a curve (a) of FIG. 11(B) and moreover no large temperature gradient is formed to the materials to be jointed in the vicinity of the electrodes. In employing the other two joining methods described earlier, if the materials to be jointed are in the same shape and of the same material, or if the joining temperature is low, there is no particular problem to be solved.
In the meantime, the electrodes 2a', 2b' for the direct resistance heating are generally formed of heat-resisting metal such as tungsten, molybdenum or the like or inorganic material with heat-resistance such as carbon or the like. These kinds of material have a good electrical conductivity and a good thermal conductivity. For instance, if the resistance values of the materials to be jointed are the same, the heat generated at the materials to be jointed between the electrodes 2a' and 2b' during the electric current supplying leaks away to an electrode tool (not shown) and a part of the materials to be jointed except the materials between the electrodes 2a' and 2b' because of the thermal conduction. As a result, a sharp temperature gradient is brought about in a longitudinal direction in the vicinity of each material to be jointed where the corresponding electrode is mounted, as is shown in FIG. 12. The temperature gradient is said to become larger as the joining temperature becomes higher, the heating speed is faster, or the heat capacity of the electrodes or the thermal conduction of the material of the electrodes is increased.
In general, it is rather strongly needed to join the materials of different materials or in different shapes and sizes at a high joining temperature, or to join different kinds of materials with a large difference of resistance values at a high joining temperature. In such cases as above, however, the heat generated at materials each to be jointed becomes different, thereby causing a problematic temperature gradient due to the flow of the heat in the vicinity of the butted portion. In the example of FIG. 11(A), supposing that the resistance values R1, R2 of the ceramics 1a', 1b' are greatly different (R1&gt;&gt;R2), the heat generated at the ceramics 1a' not only heats the butted portion, but runs away to the ceramics 1b' and the electrode tool (not shown). As a consequence, a large temperature gradient is formed in the longitudinal direction of the material to be jointed at the center of the butted portion in addition to the temperature gradient generated at each material to be jointed where the electrode is mounted, as represented by a curve (b) in FIG. 11(B). The temperature gradient is regarded to be larger as the difference of the resistance values between the materials to be jointed becomes larger, the joining temperature is higher, the heating/cooling speed is faster or the heat capacity of the ceramics 1b' is larger. When the ceramics 1a', 1b' are heated according to the method by a high frequency inductive heating under the same conditions as above, contrary to the above, the heat is generated concentrating on the ceramics 1b' of a lower resistance, assuming a temperature profile shown by a curve (c) of FIG. 11(B) which is as large as the curve (b).
The thermal stress is increased in accordance with an enlargement of the temperature gradient. When the stress exceeds the breaking strength, i.e., breaking stress of the materials to be jointed, the materials to be jointed crack and break up. If the joining temperature is raised when the temperature gradient is large, the maximum temperature of the material to be jointed which generates more heat than the other material to be jointed or the peak temperature becomes much higher than the temperature of the butted portion. Therefore, a part of the ceramics at the maximum temperature may be disadvantageously decomposed or the similar deterioration of material may be brought about. This restriction of the joining temperature is an obstruction to the use of a heat-proof material, etc. Moreover, the distance between the electrodes is difficult to be shortened and the heating speed cannot be accelerated, whereby a further electric energy is required for joining and the running cost is increased.
In the combined use of the method by a direct resistance heating and method by a high frequency inductive heating, the ceramics of a higher resistance is processed through the high frequency inductive heating so as to lower the resistance value thereof (to increase the electric conductivity) before the direct resistance heating is performed. This prior art has been employed to lower the resistance values of ceramics materials with an aim to increase the electric current for the case where the ceramics of high resistance values at room temperatures are electrically jointed together. Therefore, if the technique is applied to electric joining of two materials to be jointed of different resistance values, since the resistance between the electrodes is determined by the resistance value of the ceramics of a higher resistance, it is necessary to heat the ceramics of the higher resistance. However, when a section astride the two ceramics is subjected to the high frequency inductive heating, the ceramics of a lower resistance is mainly heated, and the electric power for the high frequency inductive heating should be increased in order to heat the ceramics of the higher resistance. If the electric power is raised, the ceramics of the lower resistance is heated further and the temperature gradient of the two ceramics becomes large. The ceramics may be broken. Therefore, if the high frequency inductive heating is used with the direct resistance heating for the purpose of increasing the electrical conductivity without considering the temperature gradient, generation of cracks due to the thermal stress resulting from the temperature gradient cannot be avoided.